Spectral beam combination (SBC) is a promising concept for combining multiple lower power lasers to form a single, higher power beam. In SBC, each low power laser element emits a discrete wavelength beamlet, and the beamlets from the multiple lasers are combined using a spectrally dispersive element to form a monolithic output. This approach preserves the beam quality (BQ) of the individual beamlets, while enabling higher power scaling than can be obtained from single elements. Generally, beam quality is an assessment of how tightly a laser beam can be focused, and a diffraction-limited Gaussian beam achieves the best possible BQ. Various metrics are used to express beam quality, such as the beam parameter product (BPP), which is the product of the beam radius at the beam waist with the far-field beam divergence, the M2 factor, which is the ratio of the beam's (second moment) bandwidth in the spatial frequency domain to the corresponding bandwidth of a Gaussian beam having the same beam radius at the waist, and the inverse M2 factor.
Prior art SBC techniques typically require that each beamlet exhibits narrow spectral linewidth to prevent loss of BQ due to angular dispersion from the combining element. However, modern high-power (i.e., multi-kilowatt class) single-spatial-mode fiber lasers typically exhibit bandwidths on the order of 1 nm (˜300 GHz), which prevents their ready use in SBC architectures. FIG. 1 depicts a prior art SBC concept that combines the outputs of wide bandwidth linear fiber laser array 10 into a combined, output beam 60 by focusing the discrete wavelength beams 12 onto a diffraction grating 40 using a Fourier transform lens 30 placed a focal distance f from array plane 12. Upon diffraction from grating 40, the discrete wavelength beams 12 propagate in the same direction and are combined into a single output beam 60. An optional output coupling mirror 50 can provide wavelength-selective feedback to each fiber in the array 10, although if the fiber lasers are configured as amplifiers with independently determined wavelengths then the output coupling mirror 50 is not needed. If the discrete wavelength input beams 12 are of narrow spectral bandwidth, then this system will increase the spatial brightness of the combined beam 60 over the brightness of the discrete wavelength beams 12. However, if the discrete wavelength input beams 12 are of broad spectral bandwidth, this increase in spatial brightness may be partially or completely offset by the reduction of spectral brightness due to the angular dispersion caused by diffraction grating 40. The broader the spectrum of each fiber laser element in the array 10, the more the spectral brightness—and, thus, the overall BQ of the combined beam 60 will be worsened by angular dispersion.
U.S. Pat. No. 7,199,924 discloses a method to overcome the angular dispersion imposed by SBC of broad bandwidth lasers by using two parallel gratings in series. The first grating imposes angular dispersion on an array of beamlets, resulting in their overlap on the second grating which removes the angular dispersion so that the beamlets are overlapped and co-propagating. While this method does indeed eliminate angular dispersion, it does so at the cost of transversely displacing the spectral content of each beamlet at the combination plane. This is known as imposing “spatial chirp” on the beamlet. This spatial chirp increases the beamlet spot size on the second grating without the concurrent decrease in the beamlet divergence that would normally occur during magnification. It can thus be seen that the BPP increases, corresponding to a loss of BQ. The loss of BQ from the spatial chirp is equivalent to the loss of beam quality that would have occurred due to angular dispersion.
While progress continues to be made in narrowing the bandwidth of high power fiber lasers, the present state of the art for narrow-band output, e.g., a few GHz, is below 1 kW. Accordingly, an improved SBC technique that combines the outputs of multiple, broad bandwidth lasers into a single beam that exhibits substantially the same BQ as each constituent beam is needed to combine fiber lasers with greater than 1 kW output powers, or equivalently to relax the requirement for narrow spectral bandwidth on the input fiber laser array at any power level.